Exploiting Variable Impedance in Domains with Contacts

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The control of complex robotic platforms is a challenging task, especially in designs with high levels of kinematic redundancy. Novel variable impedance actuators (VIAs) have recently demonstrated that, by allowing the ability to simultaneously modulate the output torque and impedance, one can achieve energetically more efficient and safer behaviour. However, this adds further levels of actuation redundancy, making planning and control of such systems even more complicated.
VIAs are designed with the ability to mechanically modulate impedance during movement. Recent work from our group, employing the optimal control (OC) formulation to generate impedance policies, has shown the potential benefit of VIAs in tasks requiring energy storage, natural dynamic exploitation and robustness against perturbation. These approaches were, however, restricted to systems with smooth, continuous dynamics, performing tasks over a predefined time horizon. When considering tasks involving multiple phases of movement, including switching dynamics with discrete state transitions (resulting from interactions with the environment), traditional approaches such as independent phase optimisation would result in a potentially suboptimal behaviour.
Our work addresses these issues by extending the OC formulation to a multi-phase scenario and incorporating temporal optimisation capabilities (for robotic systems with VIAs). Given a predefined switching sequence, the developed methodology computes the optimal torque and impedance profile,
alongside the optimal switching times and total movement duration. The resultant solution minimises the control effort by exploiting the actuation redundancy and modulating the natural dynamics of the system to match those of the desired movement. We use a monopod hopper and a brachiation system in numerical simulations and a hardware implementation of the latter to demonstrate the effectiveness and robustness of our approach on a variety of dynamic tasks.


Optimal Control of Variable Stiffness Policies: Dealing with Switching Dynamics and Model Mismatch.
A. Radulescu, J. Nakanishi, D. J. Braun and S. Vijayakumar
Springer Tracts in Advanced Robotics: Geometric and Numerical Foundations of Movements, 2017
Spatio-temporal stiffness optimization with switching dynamics.
J. Nakanishi, A. Radulescu and S. Vijayakumar
Autonomous Robots, 2016
Optimal Control of Multi-Phase Movements with Learned Dynamics.
A. Radulescu, J. Nakanishi and S. Vijayakumar
International Conference on Man-Machine Interactions (ICMMI), 2016
Spatio-temporal optimization of multi-phase movements: Dealing with contacts and switching dynamics.
J. Nakanishi, A. Radulescu and S. Vijayakumar
International Conference on Intelligent Robots and Systems (IROS), 2013
Exploiting Variable Physical Damping in Rapid Movement Tasks.
A. Radulescu, M. Howard, D.J. Braun and S. Vijayakumar
International Conference on Advanced Intelligent Mechatronics (AIM), 2012